Although the steel industry is a highly energy-consuming industry that accounts for roughly 10% of the total amount of energy consumed in Japan, roughly 40% of the continuous blast furnace steel production process is unused waste heat. This includes sensible heat of high-temperature unrefined coke oven gas (COG) (to be referred to as “crude COG”) generated from coke ovens as a heat source that is easily recovered but has conventionally not been used. Methods consisting mainly of indirect heat recovery have been previously proposed in Patent Document 1 and Patent Document 2 as techniques for recovering the sensible heat of crude COG, and a method has been disclosed that consists of providing a heat transfer tube within a coke oven riser tube or between a riser tube portion and a gas collecting tube portion and recovering sensible heat by allowing a heating medium to circulate and flow through the inside of the heat transfer tube. In these methods, however, adhesion of tar, light oil and the like and densification attributable to carburization and aggregation progress incidental to the generated COG on the outer surface of the heat transfer pipe, resulting in unavoidable problems of decreases in heat transfer efficiency and heat exchange efficiency over time. A method has been disclosed in Patent Document 3 as a technology for solving these problems that consists of coating a catalyst such as crystalline aluminum silicate or crystalline silica onto the outer surface of a heat transfer pipe and decomposing tar and other adhered substances into low molecular weight hydrocarbons by means of the catalyst in order to maintain stable heat transfer efficiency. However, this method also does not leave the realm of an indirect heat recovery technology for crude COG sensible heat, and there are no considerations given whatsoever as to whether or not decomposition products of tar and other heavy hydrocarbons become light hydrocarbons that are easily utilized as gas fuels and the like. Moreover, the effects of deterioration of decomposition activity overtime caused by catalyst-poisoning sulfur compounds such as highly concentrated hydrogen sulfide contained in the crude COG have also not been examined.
On the other hand, a compound system combining gas turbine combined cycle (GTCC) power generation and another plant has been proposed with respect to so-called thermal energy-chemical energy conversion technology by which thermal energy, which greatly changes in quality according to temperature, is converted to chemical energy, and examples of such have been sporadically observed, including combining with oxygen production using a high-temperature oxygen-transporting solid electrolyte (Patent Document 4), and water vapor reforming and hydrogen production of natural gas using the sensible heat of gas turbine outlet exhaust gas, and its utilization as fuel (Patent Document 5). In each of these technologies, thermal energy is converted to chemical energy in the form of oxygen or hydrogen by allowing air or natural gas are to act through a functional material in the form of a solid electrolyte or catalyst.
There are hardly any technologies for converting to chemical energy by directly introducing a chemical reaction into a reactive gas formed at high temperatures in the presence of a catalyst by using the sensible heat thereof, and in nearly all cases in the prior art, sensible heat of high-temperature gas has been either recovered indirectly or not utilized at all, while only using the cooled gas after subjecting to various treatment. However, even though crude COG has sensible heat, since the content of sulfur compounds exceeds 2000 ppm, it is considered to be extremely difficult to realize from the viewpoint of designing a catalytic reaction for thermal decomposition of heavy hydrocarbons such as tar, and although studies have been made in the past as described in Patent Document 6, reforming activity has not necessarily been adequate. In addition, although energy conversion catalysts are typically produced by a loading method in which an active metal species is loaded from the outside onto a porous ceramic support such as silica or alumina, in the case of these methods, it has been difficult to increase dispersibility of the loaded metal component, and there is also susceptibility to sulfur poisoning and carbon deposition, it has been difficult to produce a catalyst suitable for decomposition reactions of tar consisting mainly of condensed polycyclic aromatic compounds that is susceptible to the occurrence of carbon deposition in an atmosphere containing highly concentrated sulfur compounds as described above. In addition, as a result of air combustion for the purpose of regeneration after performance has deteriorated following the reaction, sintering (coarsening) of the loaded metal granules occurs easily, thereby making it difficult to realize restoration of activity by regeneration.
In addition, with respect to production methods of mixtures of nickel-magnesia compounds and alumina as well, if a nickel-magnesia compound powder and an alumina powder are simply mixed followed by molding and baking, the state in which each element is present in the catalyst is not uniform, and the active species nickel in particular ends up coagulating without reaching a high surface area resulting in problems such as inadequate reforming activity and large amounts of deposited carbon, thereby preventing these methods from reaching a level able to withstand practical use.
On the other hand, although methods for producing oxides containing nickel, magnesium and aluminum are disclosed in publications such as Non-Patent Document 1 and Patent Document 8 with respect to materials that are baked after having formed a precipitate (mainly the formation of a hydrotalcite structure) with a precipitating agent from an aqueous solution in which each metal component has been dissolved, these methods had problems with practical application due to inadequate reforming activity and large amounts of deposited carbon.
As a result of conducting extensive studies with the foregoing in view, the inventors of the present invention determined that a catalyst produced according to a method consisting of forming a co-precipitate with a precipitating agent from an aqueous solution containing a nickel component and a magnesium component, drying and calcining the co-precipitate, and then drying and baking or drying, calcining, molding and baking a mixture obtained by adding alumina powder and water or an alumina sol, demonstrates high reforming activity and demonstrates comparatively little carbon deposition, thereby leading to the filing for patent (Japanese Patent Application No. 2008-155887). However, it was considered to be necessary to further reduce the amount of deposited carbon in order to develop a catalyst that demonstrates stable activity over a long period of time for the purpose of practical application.
Moreover, attention has recently been focused on the use of a biomass, which is a kind of carbonaceous raw material, as effective means of reducing carbon dioxide emission levels due to the problem of global warming, and research relating to highly efficient energy conversion of biomass is being conducted at various facilities. In addition, from the current viewpoint of securing energy resources, research relating to effective utilization of coal, which has been aggressively conducted in the past, is also being reconsidered for practical application. Among this research, although various studies have been conducted, including that described in Patent Document 7, on methods for generating crude gas (unrefined gas) by gasifying tar formed in the dry distillation of biomass and then utilizing its sensible heat while focusing particularly on reforming catalyst of tar using a catalyst, this approach has not always been adequate from the viewpoints of catalyst activity and catalyst regeneration in the same manner as decomposition reactions of coal-derived tar as previously described.